A power module is a device for which a pair of switching elements are connected in series to a power supply, and which obtains an output from between the pair of switching elements. Such a power module is used for, for example, an inverter circuit that forms a driving circuit to drive an electric motor. The electric motor is used as, for example, a power source of an electric vehicle (including a hybrid car), a train, an industrial robot, and the like. The power module is also applied to an inverter circuit that converts electric power generated by a solar battery, a wind power generator, and other power generators (particularly, a private electric generator) so as to be consistent with the electric power of a commercial power supply.
For the switching elements of power modules, devices using Si (silicon) semiconductors have been conventionally used. However, there has been a problem of losses in the devices at the time of power conversion, and the situation is that a further improvement in efficiency of the devices using Si materials is no longer feasible.
Therefore, a power module using as its switching elements power devices using SiC (silicon carbide) semiconductors has been proposed. The SiC power devices are capable of a high-speed ON/OFF operation because the switching speed is high. Consequently, a current quickly decreases at the time of switch-off, so that switching loss can be reduced.
However, high-speed switching by the SiC power devices causes a new problem that an increase in surge voltage at the time of switching occurs.
The surge voltage V is, as shown in the following formula (A), given by a product of a self-inductance L which internal wiring of the power module has and a differential (di/dt) of a current i by a time t (a current change ratio per hour).V=L·(di/dt)  (A)
The higher the switching speed, the greater the change ratio (di/dt) of the current i, so that the surge voltage V is increased. When the surge voltage loads the devices with a voltage not less than a breakdown voltage, the devices may be broken. Moreover, when the surge voltage is great, there are also concerns of an increase in EMI (electromagnetic interference) noise and a reduction in reliability.
Therefore, in order to reduce surge voltage while applying high-speed switching elements such as SiC devices, it is necessary to reduce the self-inductance L which the internal wiring of the power module has. This challenge is common not only to power modules but also to semiconductor devices having switching elements. Of course, also in semiconductor devices having switching elements using Si semiconductors, a reduction in surge voltage is a significant challenge.